The herpes simplex virus (HSV) is a common infection caused by two primary strains: HSV-1, which historically caused oral herpes (cold sores), and HSV-2, which is the most frequent cause of genital herpes. While the clinical presentation of an outbreak may suggest a diagnosis, laboratory testing is needed to confirm the presence of the virus and determine its specific type. A swab test is a diagnostic tool used to identify an active HSV infection by collecting material directly from a suspicious lesion.
The Purpose of Swab Testing for Herpes
The purpose of a swab test is to capture the herpes virus during a period of active replication, which happens when a visible sore is present. A healthcare provider will collect the sample by gently rubbing a sterile swab against the base or surface of a blister, ulcer, or open sore. The goal is to collect fluid and cells that contain the highest concentration of viral particles.
The timing of this collection is a significant factor affecting the test’s accuracy, as the highest viral load is present in new lesions. Ideally, the sample should be collected from a fresh blister or ulcer, often within 48 hours of its appearance. As a lesion begins to heal or crust over, the amount of viable virus decreases rapidly.
Viral Culture Versus PCR Testing
Once a swab sample is collected, it is sent to a laboratory where it is processed using one of two primary methods: viral culture or Polymerase Chain Reaction (PCR) testing. Viral culture is the older technique, which involves placing the collected cells into a special medium to see if the virus can grow and multiply. This method is highly specific, meaning a positive result is reliable, but it is less sensitive and can take several days to produce a result.
Viral culture frequently results in a false negative if the lesion is already beginning to heal or if the sample was poorly collected. PCR testing looks for the unique genetic material (DNA) of the herpes virus. PCR works by amplifying even minute amounts of viral DNA, making it significantly more sensitive than viral culture, especially for older or recurrent lesions.
PCR is also substantially faster, often providing results within hours or a single day, and is now considered the standard for testing active lesions due to its superior ability to detect low viral concentrations, making it the most reliable method for confirming an active infection.
Interpreting Swab Test Outcomes
A positive result from a swab test, particularly one processed by PCR, confirms that the observed lesion is caused by the herpes simplex virus. Because these tests directly detect the virus’s presence or genetic code, a positive outcome is reliable. Furthermore, a swab test can often distinguish between HSV-1 and HSV-2, which can be valuable information for management and counseling.
A negative swab test outcome, however, does not always guarantee the absence of herpes infection. The most common limitation of swab testing is the potential for a false negative result, which can occur in a wide range of 25% to 70% of cases depending on the testing method and lesion stage. This happens when the sore has crusted over or healed to the point where there is no longer enough active virus present. Therefore, a negative result must be interpreted in the context of when the lesion first appeared and how the sample was collected.
When Blood Tests Are Used Instead
While swab tests diagnose an active outbreak, blood tests are used to determine if a person has ever been exposed to the herpes virus, even if they have never shown symptoms. Blood tests, known as serology tests, detect antibodies that the immune system produces in response to an HSV infection. The two main types of antibodies measured are IgG and IgM.
A positive IgG antibody test confirms a past exposure to the virus, indicating a latent or dormant infection, not necessarily a currently active outbreak. Blood tests are the preferred method for screening asymptomatic individuals or confirming a diagnosis when lesions have already healed. Since the body can take up to 12 weeks to produce detectable levels of antibodies, a blood test taken too soon after a suspected exposure can also lead to a false negative result.